The present invention relates generally to motor control and, more particularly, to a position feedback device that predicts position for incoming position data requests that occur between position updates.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Rotating motors are typically controlled by a motor drive that receives a reference motor velocity signal and, based on the motor velocity signal, produces and outputs a torque signal that is applied to the motor. Adjustment of the torque signal based on changes to the reference velocity signal relative to a feedback velocity signal ensures that the motor rotates at the reference velocity.
Some applications require precise motor control across multiple, synchronized motors. For example, an electronic line shaft may be employed in a printing application to move the paper or other material over rollers and through various stages of the printing process. Typical printing processes employ multiple colors, each applied at different locations along the line. Hence, to ensure print quality, the various stages are synchronized. A lack of synchronicity between the stations results in misregistration between the colors, leading to unacceptable product that may need to be scrapped.
Previous generations of printing technology employed a mechanical line shaft mechanically linked to the various printing stations. Rotation of the line shaft by an electric motor activated rollers and other printing station tools along the line to conduct the printing process. In a mechanical line shaft system, factors such as play in the mechanical linkages, stretching of the paper web, and torsional flexing of the line shaft itself make it difficult to achieve and maintain synchronicity between the printing stations, especially during periods of acceleration and deceleration of the printing system. It has been observed that when synchronicity is not maintained, product generated includes excessive flaws and is often unacceptable for intended use. Mechanical line shafts also have reduced flexibility in addressing print changes. Hence, where changes are required, down time may be excessive.
More modern printing systems, commonly referred to shaftless printing systems or electronic line shaft systems, employ a plurality of motors and associated rollers that are electrically synchronized, as opposed to mechanically synchronized. Lack of synchronicity in an electronic line shaft results in similar problems, such as color misregistration, evident in a mechanical line shaft system.
When operating a plurality motors synchronously in an automated system, several factors exist that may cause the position of the motors to deviate from each other even though they are all operating pursuant to a single reference velocity signal. For instance, motor inertia between motors at different stations is often non-uniform and can cause one motor to drift from the other motors.
Position errors in a drive system are controlled by a position regulator that acts on the difference between a reference position and a feedback position determined using a position feedback device such as, for instance, an optical encoder. That difference is commonly referred to in the motor control industry using terms such as “following error”, “tracking error”, and “position error”. The resolution of the position feedback device determines the number of discrete position references by which the position of the drive may be controlled.
One known position feedback device, commonly referred to as a Heidenhain encoder scans optical markings disposed about the periphery of a disk that rotates with the load. The encoder generates a two-channel output, one being a sine wave and the other a cosine wave. Typically, these channel signals are passed through filtering circuitry to convert them to square wave or edge signals. The edges are counted and used as position references for determining the rotational position of the drive. The edge counts are stored in a counter, such that forward motion increments the counter while reverse motion decrements the counter. The speed of the motor is typically determined by comparing the counter values over a predetermined time interval and dividing the number of counts by the time interval. The value stored in the position counter may be referred to as course position.
In some applications controlling the drive based on course position provides sufficient precision. However, in other applications, a more precise position control is desired. A technique for increasing the resolution of the optical encoder involves sampling the sine and cosine signals generated by the encoder prior to converting into square waves for edge counting. The sine/cosine data provides information concerning the incremental position of the drive (i.e., position between the edges used to generate the course position). An incremental position value is determined by computing the arctangent of the sine/cosine signals to yield a fractional angular position. Thus, the position of the drive is represented by a composite value in which the most significant bits are generated by the course position stored in the counter and the least significant bits are generated using the incremental position resulting from the arctangent function. The incremental position technique can be employed to increase the resolution of the encoder by up to several orders of magnitude.
Even with the increased resolution made available using the arctangent technique, some position error still remains in the feedback signal due to the discrete nature of the hardware used to generate the position data. The course and incremental position are updated at predetermined intervals, however, typical feedback units operate asynchronously with respect to the position and velocity regulators used to control the drive. Hence, the drive unit may request position data between updates by the feedback unit. In such an instance, the feedback unit provides the position data as of the last update (i.e., sample and hold). In applications with stringent precision requirements, this error in the position feedback signal is unacceptable.
Thus, it would be desirable to provide more accurate position measurements when the feedback unit is operated asynchronously with respect to the drive regulators. In a printing application, it would be advantageous to increase the accuracy of the position feedback to ensure the quality of printed product, thereby reducing waste.